1920] 



on Crystal Structure 



193 



all directions, the diffraction of the X-rays resulting in the appearance 

 of a set of " halo " rings on a suitably placed photographic plate. He 

 analysed graphite, and showed that so-called " amorphous " carbon 

 consists in reality of minute graphite crystals. The arrangement of 

 the atoms in silicon, tungsten, tin, gold, aluminium and other ele- 

 ments has been discovered by Debye's method. Sherrer has found 

 that colloidal gold and silver consist of minute crystals, the dimensions 

 of which are so small that they are only four or five atoms deep in 

 any direction, and which yet retain exactly the same crystal structure 

 as massive gold and silver. The same method was arrived at incle- 



A ^ 2°6qA 



UNIJOF IRON PYRITES STRUCTURE 

 Fig. 1. 



pendently by Hull, and he has extended it to a number of inte- 

 resting crystals. We now know the atomic arrangement for a very 

 large proportion of the crystalline elements and simple compounds. 



2. The essential principle of these methods of crystal analysis 

 may be compared to that involved when a diffraction grating is cali- 

 brated by means of monochromatic light, the wave-length of which is 

 known accurately. By finding the angle at which the light is 

 diffracted by the grating, the distance apart of the lines ruled on the 

 grating can be calculated. The planes on which the atoms of a crystal 

 are arranged correspond to the lines of a diffraction grating. A beam 



Vol. XXIII. (No. 114) o 



